1. Field of the Invention
This invention relates to a method and apparatus for detecting and discriminating a pulse code modulated RF signal transmitted from an implanted medical device to a medical device programmer from transient and steady state noise. 2. Description of the Prior Art
In the field of programmable implanted medical devices, such as cardiac pacemakers, tachyarrhythmia-control devices, implantable drug dispensers and nerve stimulators, it has become common to provide an interactive, transceiver system for both remotely programming operating functions, modes and parameters of the implanted device, and telemetering out data related thereto on command by RF telemetry. In nearly all such active, electronic, implanted medical devices, it has become highly desirable to have the ability to reprogram the device's modes of operation, parameters and other functions and to monitor the performance of the device, both historically and contemporaneously. Such current medical devices are designed to provide two-way telemetry by radio frequency signal transmission between the implanted device and the programming head or wand of the external programmer to provide for the exchange of binary-coded transmitted information to enable the aforementioned programming by telemetry in and the reading out of data stored in the device by telemetry out.
For example, the Medtronic U.S. Pat. No. 4,253,466 describes an implantable digital, programmable, cardiac pacemaker pulse generator which may be programmed by the programmer described in Medtronic U.S. Pat. No. 4,250,884, both incorporated herein by reference. Such programmers as disclosed in the '884 patent are microprocessor-based and menu-driven under an overall operating routine and subroutines. Such programmers display operating conditions, commands and error messages to prompt the user in the proper use of the programming system. The system of the '466 and '884 patent sets forth the software-controlled programming of a series of Medtronic.RTM. implantable pacemakers (IPG's), but it does not describe the uplink telemetry out on command of the external programmer of the programmed commands, electrogram, end of-life indicators and the like that have been developed and implemented in subsequent pacing systems.
More recently, microprocessor-based programmers have been developed by Medtronic, Inc. and others which are operated under the control of dedicated, plug-in ROM modules to enable the operation of the system with regard to specific model or series of models of implanted pulse generators. In such systems, the programmer is incapable of operating until a plug-in module or cartridge has been properly installed. The Medtronic.RTM. MemoryMod.TM. cartridge enables the physician to apply the programmer to a specific set of pulse generator models. The software cartridge concept allows Medtronic, Inc. to expand and update the application of the programmer to new pulse generators and functional capabilities as they become available.
In current implanted pacemakers, confirmation of a programming transmission occurs by automatic uplink transfer of data via telemetry out. Use of an ECG cable and electrode leads is not required for programming confirmation with these devices. However, if telemetry is not received because of strong electrical interference, the programmer will attempt to confirm the transmission by surface detection of the PCI (program confirmation indicator) issued by the IPG. This method of program confirmation can occur only if the programmer is connected to electrodes on the patient. As described below the present invention discloses improvements which are directed toward the reduction or elimination of failures in telemetry out because of strong electrical interference.
Furthermore, advanced programming and telemetry systems envisage the transmission of the patient's electrogram directly from the pacing electrodes in contact with the heart to the programmer via the RF telemetry link in order to provide near-field and/or far-field electrograms directly from the myocardium. In such systems, the effects of electrical interference and other noise on the telemetry transmission is of concern. Medtronic U.S. Pat. No. 4,556,063 (incorporated herein by reference) describes such a system for telemetering out both digital and analog data.
The presence of electrical interference or noise strong enough to interrupt reception of telemetry from the implanted pulse generator can affect operation of and telemetry functions, including programming confirmation.
Hospital operating rooms, catherization laboratories and even physicians' offices are often noisy electrical environments, and such noise has been found on occasion to interfere with the proper programming or interrogation of an implanted pacemaker. Such programming and interrogation of the implanted pacemaker's function is commonly undertaken as part of the surgical implant of the implanted pulse generator. Other equipment in the operating room or in adjoining rooms or floors of the facility may generate severe electrical noise. To ensure the safety of the patient, prior systems, as explained above have been designed conservatively to avoid misprogramming once acquisition has been obtained by closure of a reed switch within the implanted pulse generator.
In the prior art systems referenced above, the physician was instructed to either identify the source of interference and eliminate it or move the patient and programmer to another location to avoid its effects. In addition, software controllable gain control circuitry was provided which attempted to optimize the gain in the presence of interference to pick up only telemetered-out RF signals and exclude the noise or interference.
In response to the above-described disadvantages of the prior art systems, attempts have been made to provide automatic gain control circuitry (see for example, U.S. Pat. No. 4,562,840 and software subroutines e.g., those within the Medtronic.RTM. MemoryMod cartridges, and specifically disclosed in Medtronic U.S. Pat. No. 4,531,523, incorporated herein by reference.
The Maltronic.RTM. 9710 programmer and improved models of that programmer provided circuitry of the type depicted in Medtronic U.S. Pat. No. 4,542,532, for detecting a telemetry signal by use of a receiver input circuit tuned to the 175 kHz center frequency and a bandpass filter and manual gain control. The signal developed in the receiver circuit is further processed in conjunction with the software implemented gain control referenced above. The transceiver of the '532 Patent implemented, a dual antennae, to reject essentially far-field noise which tends to link both coils with the same field strength. As explained in the '532 Patent, since, in the receive mode, the two coils are wound in series opposition, the noise field component from the remote noise source would be cancelled, leaving primarily the signal component at the input of the receiver.
However, deficiencies in the filtering circuit of the '532 Patent as implemented in the Model 9710 series programming system have continued to cause difficulty in the reception of radio frequency pulse code modulated signals transmitted from the implanted pacemakers.
The problems with artifact noise have been addressed with software algorithms as referenced above. However, it is desirable to so design the circuitry as to substantially reduce the remaining problems with transient and steady state noise.